Sending Mail Via Postfix Woes

Hello there. I recently installed ubuntu and ispconfig3 following one of those "perfect server" guides. Everything appeared to go smoothly. However, when connecting to my email accounts on the server through thunderbird, I can receive email but cannot send email. I can send and receive email via the webmail webpage. Postfix is operating on port 587 as I'm on comcast and that's what they require for outgoing mail. The error that I get in /var/log/mail.log:

Sending of message failed.
The message could not be sent because the connection to SMTP server mail.domain.com [yes, this has my real domain not a dummy] was lost in the middle of the transaction. Try again or contact your network administrator.

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I imagine that it's a misconfiguration of dovecot or postfix but as I'm new to linux and system administration, I have no clue regarding how to fix the issue or how to go about troubleshooting the issue.

You are not showing any log lines that would be related to an incoming SMTP connection from an external host. On the other hand, are you sure that the name 'mail.domain.com' (replace with your name) gets mapped to your server? You said you can receive mail but there are no connections from outside in your log snippet.

This is a server on my home LAN. I was running thunderbird from another machine in my LAN. As far as relevant log lines, what log should I post the contents of (or at least a cat | tail )? I'm new to linux and server administration and started this as a hobby so any guidance is greatly appreciated. My mynetworks parameter in /etc/postfix/main.cf which I believe might be relevant is

mynetworks = 127.0.0.0/8 [::1]/128

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I have tried adding 192.168.1.0/24 to this variable thinking that perhaps no auth attempts were made because they never made it to the auth daemon due to being blocked by this parameter and noticed no difference. I have since reverted my addendum.

Whoops. I didn't realize I had changed my username. Thanks for pointing that out. I did as you have instructed and here are the results:

Thunderbird's error:

Sending of message failed.
The message could not be sent because connecting to SMTP server 192.168.1.9 failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server settings are correct and try again, or contact the server administrator.

Wow! Thank you so much Sir Henry! I tried sending on port 25 and it worked and I enabled submission on 587 and it worked there. Thank you for the support. Just one thing, can you summarize what the problem was for me? What I gather is that postfix wasn't able to accept email submissions on port 587 by default so my connections from thunderbird were being dropped by postfix. Is this correct?